Plaggen soil is a special kind of soil that is made by people a long time ago. They would take turf, which is like thick grass or weeds that grow in a swamp or marsh, and cut it into large pieces. Then they would lay these pieces of turf on top of the soil in fields where they grew crops.
Over time, the turf would start to decay and break down, turning into a kind of black soil that was very good for growing crops. The new soil was also full of nutrients that plants needed, like nitrogen and phosphorus.
This method of making soil was used a lot in Europe hundreds of years ago, especially in places where the soil was thin or poor quality. Nowadays, farmers don't really use plaggen soil as much anymore, since there are other ways to improve the quality of soil. However, some people still think it's a cool and interesting way that people used to farm!